1. Field of the Invention
The present invention relates generally to methods and apparatus for conducting electrostatic spray coating, and more particularly, is directed to an improvement in a method of providing an object to be coated with a coating layer thereon through spraying coating material by a rotary spray head device and depositing electrostatically the sprayed coating material on the object, and also to an apparatus for use in such an improved method.
2. Description of the Prior Art
There has been proposed adoption of electrostatic spray coating for providing a vehicle body with a coating layer which covers uniformly the whole outer surface of the vehicle body. Generally, according to the electrostatic spray coating, an electrostatic field is formed with a high DC voltage supplied between an object to be coated, such as the vehicle body, and a spray head device disposed to face the object for spraying coating material toward the same, and the coating material sprayed by the spray head is charged with electricity in the electrostatic field and deposited on the object through electrostatic adsorption effected thereon. With use of such electrostatic spray coating the efficiency of deposition of the coating material on the object is improved so that a loss of coating material is reduced.
When the object which is subjected to the electrostatic spray coating is made of metal, coating of the baking finish type, such as one of various kinds of coating of melamine alkyd resin, is ordinarily used for forming a coating layer on the object. The coating of the baking finish type deposited on the object is dried in a heated ambience of, for example, about 140 degrees to be hardened. On the other hand, when the object which is subjected to the electrostatic spray coating is made of some material with relatively low thermal resistance such as a kind of plastic, a coating of the two liquid component type, which comprises a coating base of, for example, acrylic resin and a hardening agent, is used for forming a coating layer on the object. The coating of the two liquid component type disposed on the object is dried in an ambience of ordinary temperature to be hardened, so that the coating layer which is expected to be securely deposited on the outer surface of the object to have a surface thereof with superior gloss, hardness and weather-tightness, is formed on the object.
In general, such coating of the two liquid component type as described above is hardened in a relatively short time when the coating base and the hardening agent are mixed with each other, and therefore the coating base and hardening agent are stored separately and mixed with each other immediately before the electrostatic spray coating is conducted. Therefore, when electrostatic spray coating is conducted with the coating of the two liquid component type, a mixer is provided for mixing the coating base with the hardening agent to constitute the coating of the two liquid component type, and the coating sent out of the mixer is supplied to a spray head device so as to be sprayed thereby. Accordingly, when a change of color of the coating base is required, the mixer is to be caused to discharge the coating produced therein to the outside therefrom and to be washed thoroughly to remove the remaining coating material from the inside thereof before a new coating base of different color is supplied to the mixer together with the hardening agent. This results in a problem in that a relatively long time is wasted for washing the mixer in a sequential coating process.
For the purpose of avoiding such a problem, there has also been proposed an apparatus for performing the electrostatic spray coating without using a combination of the mixer for mixing the coating base with the hardening agent to constitute the coating of the two liquid component type and the spray device for spraying the coating, as disclosed in the Japanese patent application published before examination under publication number 57-45370 or 59-228960. Such a previously proposed apparatus is provided with a rotary spray head device which is supplied with a high DC voltage and operative to rotate at a high speed, and first and second nozzles for supplying respectively the coating base and hardening agent, which are ingredients of the coating of the two liquid component type, separately onto the inner wall surface of the rotary spray head. The coating base and hardening agent supplied onto the inner wall surface of the rotary spray head device are mixed with each other in the rotary spray head device and sprayed from the rotary spray head device due to centrifugal force resulting from the rotation of the rotary spray head device.
Then, the previously proposed apparatus is further provided with a third nozzle for supplying a washing agent, such as thinner, to the inside of the rotary spray head device. The third nozzle is disposed to have its discharging opening positioned in the vicinity of the inner wall surface of the rotary spray head device.
When the change of color of the coating base is carried out, the third nozzle is operated to supply the washing agent onto the inner wall surface of the rotary spray head device for washing the inside of the rotary spray head device after the first and second nozzles are controlled to cease to supply the coating base and hardening agent, respectively, so that the rotary spray head device is made ready for the change of color of the coating base. With such an arrangement, a time spent for the change of color of the coating base is effectively saved.
When the electrostatic spray coating is conducted with the above described apparatus which is equipped with the rotary spray head device and the first, second and third nozzles for supplying the coating base, hardening agent and washing agent, respectively, each of the coating base and hardening agent supplied onto the inner wall surface of the rotary spray head device reaches to the open end portion of the rotary spray head device so as to be sprayed therefrom in an instant, due to the centrifugal force resulting from the rotation of the rotary spray head device. Accordingly, it is feared; that the coating base and the hardening agent which ar supplied separately onto the inner wall surface of the rotary spray head device are sprayed independently from the open end portion of the rotary spray head device, and therefore the coating base and hardening agent both sprayed by the rotary spray head device are not mixed enough with each other. In the case where the coating base sprayed by the rotary spray head device is not mixed sufficiently with the hardening agent also sprayed by the rotary spray head device, an object to be coated through the electrostatic spray coating is provided on its outer surface with a coating layer which has partially excess and deficiency of the coating base or the hardening agent, and this results in a disadvantage, in that the coating layer deposited on the outer surface of the object is lacking in uniformity in gloss, hardness, weather-tightness, and other characteristics.